Well this is the first time in my internet history I cant locate a rego screen.
Not many other products runs together with so many bad-behaving add-ons like FSX, so the registration screen is not coming because one of these products is forcing itself onto the foreground. It would be wrong if we tried to do the same questionable behavior (forcing our dialog in the foreground), just to be sure the registration can be always be seen, even something so misbehaving is running.
Its windowed
Of course it's windowed, the registration won't even allow you to try, if you are not running windowed.
and launched from FSX and no other add ons interfering , nothing different from other FSDT installations.
I can only repeat and confirm, if you click the Register Serial and nothing happens, it's SURE another program is interfering. The registration dialog is probably there, but you can't see it, because that product is forcing itself onto the foreground. You might try to bring the registration dialog back in front of the FSX window, using the CTRL+TAB or ALT+TAB keys.
It's either that, or you haven't started FSX from its own icon. No other reasons, if this is really happening according to your report, which is "nothing happens" after pressing the "Register Serial" button.